1. Field Of The Invention
The present invention relates, in general, to regeneration of N-methylmorpholine N-oxide used to prepare a cellulose fiber or film and, more particularly, to a method for decolorizing a used aqueous N-methylmorpholine N-oxide solution through purification of coloring impurities of an aqueous solution after spinning.
2. Description Of The Prior Art
For cellulose fiber or film, a spinning cellulose dope containing N-methylmorpholine N-oxide as a solvent is usually extruded into an aqueous coagulation solution through a spinning die with the N-methylmorpholine N-oxide being eluted, and then subjected to washing. In this procedure, there is generation of an enormous quantity of spinning aqueous solution (a coagulating solution and a washing solution) in which N-methylmorpholine N-oxide is dissolved at a low content.
The cellulose dope employing N-methyl morpholine N-oxide as a solvent is prepared at a high temperature, and the dope is heated when it is extruded through a spinning die. However, N-methylmorpholine N-oxide is apt to decompose due to the high temperature, generating coloring impurities. Consequently, the cellulose dope may be discolored by them.
Most of the coloring impurities are eluted along with N-methyl morpholine N-oxide in the coagulating and the washing processes, but some remain unremoved even by further washing processes, coloring the final articles, such as fibers or films, ultimately. Therefore, a bleaching process is additionally required.
In order to reclaim and reuse the N-methylmorpholine N-oxide which is dissolved in the spinning aqueous solution at a low content, concentration is performed, followed by purification. N-methylmorpholine N-oxide reclaimed but not purified is tinged with dark brown by the coloring impurities contained. In the case that the reclaimed N-methylmorpholine N-oxide is used to produce fibers or films without removing the coloring impurities, continuous accumulation of them comes to tinge the final articles with color, deleteriously affecting the final articles.
As a basic solution for the discoloration problem, there is use of pure cellulose (commercially available wood pulp with little coloring organic materials) and highly purified N-methylmorpholine N-oxide.
D.D. Patent No. 259,863 discloses treatment with a cation exchange resin of sulfonized divinyl benzene-styrene copolymer at pH 5-8 (H.sub.3 PO.sub.4), thereby increasing purity of an aqueous N-methylmorpholine N-oxide solution obtaining from oxidation of N-methylmorpholine.
In many patents including European Patent No. 0 047 929, DE Patent Nos. 3 034 685 A1 (Akzo) and 41 06 029 A1 (Lenzing A. -G.), PCT WO83/04415, U.S. Pat. No. 4,581,072 (Courtaulds), and D.D. Patent Nos. 229 708 A1, 218 104 A1 (VEB Chem) and 158 656 (ADW I.F.P), there are described a variety of antioxidants to suppress oxidative thermal degradation of both cellulose and N-methylmorpholine N-oxide.
European Patent No 467,008 (Lenzing A. -G.) discloses that 0.01-2% of H.sub.2 O.sub.2 along with a stabilizing additive is added to prepare a cellulose dope, thereby suppressing the oxidative thermal degradation of N-methylmorpholine N-oxide in the course of production and extrusion of the dope.
However, even though highly pure raw materials are used and the antioxidants are added as mentioned above, perfect suppression of oxidative thermal degradation is impossible. In fact, alkaline bleaching (e.g. aqueous sodium hypochlorite solution or aqueous hydrogen peroxide/sodium hydroxide solution) is carried out for the colored article, followed by further processes such as neutralization and washing.
According to PCT WO 92/14871 (Courtaulds), it is reported that the alkaline bleaching, however, causes fibrillation of fiber, leading to occurrence of defects which are fatal to dyeing property of fabric and to forming pills on its surface.
An effort has been made to purify the aqueous N-methylmorpholine N-oxide solution reclaimed after the production of cellulose fiber or film.
For example, D.D. Patent No. 254 199 A1 (VEB Chem.) suggests a purification method comprising passing an aqueous solution containing 5 to 60% by weight of amine oxide through an anion exchange resin of styrene-divinyl copolymer. According to this patent, the aqueous amine oxide solution is treated initially with a weak anion exchange resin with the amine group of --CH.sub.2 N(CH.sub.3).sub.2 and then with a resin with the tertiary ammonium group of --CH.sub.2 [N(CH.sub.3).sub.3 ].sup.+ OH.sup.-. After passing through the first resin column, the deep brown aqueous solution is purified into pale brown or yellow, which is further purified into pale yellow or colorless by the second resin column. The supra patent also says that the used resins can be regenerated by treatment with 3 wt % aqueous sodium hydroxide solution and 10 wt % aqueous sodium chloride solution. Such method can take off coloring impurities from the aqueous amine oxide solution, however, the regeneration of resin by strong alkali is unsatisfactory, so that the separation efficiency of the regenerated resin drops largely.
In addition, European Patent No. 0 427 701 A1 (Lenzing A. -G.) discloses use of a resin with --CH.sub.2 [N(CH.sub.3).sub.3 ].sup.+ X.sup.- or --CH.sub.2 [N(CH.sub.3).sub.2 (CH.sub.2 OH)].sup.+ X.sup.- group, wherein X.sup.- is an anion of organic or inorganic acid. The resin is regenerated by a volatile acid, such as hydrochloric acid, carbonic acid, formic acid and acetic acid, which in turn is reclaimed by distillation. The resin regenerated is able to purify the aqueous amine oxide solution and thus, to give decolorized solution in a high efficiency. However, when strong inorganic acid, e.g. hydrochloric acid is employed, there is an disadvantage that apparatus is highly apt to be corroded because hydrochloric acid or a salt of Cl.sup.- is eluted along with the aqueous solution.
Unlike two supra patents using either alkali or acid to regenerate the resin, PCT WO92/11287 (Courtaulds) suggests use of both alkali and acid stepwise to regenerate resin, thereby more efficiently removing the impurities adsorbed on the resin. According to the patent, an anion exchange resin with --CH.sub.2 [N(CH.sub.3).sub.3 ].sup.+ X.sup.- or --CH.sub.2 [N(CH.sub.3).sub.2 (CH.sub.2 OH)].sup.+ X.sup.- group which has purified the reclaimed aqueous amine oxide solution is treated initially with an aqueous 1-10 wt % hydrochloric acid or sodium chloride solution to take off most of coloring impurities with the X.sup.- ions of resin being changed into Cl.sup.- ions, and then treated with an aqueous 1-10 wt % sodium hydroxide solution with X.sup.- ions of resin being changed into OH.sup.- ions. In the second treatment, sodium hydroxide is used in the same equivalent as the strong acid or in a little larger equivalent (mole ratio 1.0-1.1) to elute a neutralized solution containing only sodium chloride, thereby preventing the corrosion of apparatus.
All the described methods using ion exchange resins can purify the aqueous amine oxide solution and thus give a decolorized solution, but generates a strong acidic solution, a strong alkaline solution or a salt solution containing coloring impurities during the regeneration of resin. To prevent the pollution by these waste solution, re-treatment processes such as concentration and incineration are required. In addition, since the resin is not lasting but has a life span of 2-3 years, equipment and its operation cost too much. Further, because amine oxide as well as the coloring impurities is well absorbed into the resin, the loss of amine oxide is considerable at high concentration. For example, when an aqueous 62 wt % amine oxide solution is treated with anion exchange resin, an aqueous 43 wt % amine oxide solution is obtained after purification and decolorization. Furthermore, as the concentration of amine oxide is higher (e.g. not less than 40%), the absorption efficiency of resin for the coloring impurities becomes low steepy. This is a main obstacle that restrains the resin from being used to purify an aqueous amine oxide solution with a high concentration. So, there is a troublesome problem that the resin should deal with more increased quantity of aqueous amine oxide solution diluted into a lower concentration.
In general, active carbon rather than the ion exchange resins is frequently used to remove the coloring impurities because of its cheapness. It is well known that active carbon is used in two forms: bead and powder. Active carbon bead capacitates continuous work for taking off the coloring impurities. In addition, used active carbon bead can be regenerated, like the ion exchange resin. But, it is low in adsorption rate and has difficulty in yielding a high purity. On the other hand, active carbon powder is superior in adsorption rate and capable of purifying the aqueous amine oxide solution in high degrees in batch type. However, active carbon powder is uneconomical because the used cannot be regenerated. Further, active carbon powder does not allow continuous work.
In purifying the aqueous amine oxide solution, active carbon bead can treat it successively and be used repetitively through regeneration. Active carbon bead, however, is remarkably low in decolorizing power as compared with the anion exchange resin. Especially, the coloring impurities contained in a high concentration of the aqueous amine oxide solution is not readily absorbed into the active carbon bead relative to on the anion exchanger.
By contraries, active carbon powder is much superior to the anion exchanger in decolorizing and purifying the reclaimed amine oxide at high content as well as at low content, even if it is less economical than active carbon bead.
Meanwhile, it is reported in Cellulose Chem. Technol. 20, pp. 289-301 (1986) to H. Lang et al that when a cellulose dope containing N-methylmorpholine N-oxide is subjected to high temperature, both cellulose and N-methylmorpholine N-oxide are decomposed, mainly giving N-methylmorpholine, morpholine and carbon dioxide. It is also reported that N-methylmorpholine amounts up to 85% of the decomposed products from N-methylmorpholine N-oxide.
Das Papier. 40 (12), pp 615-619 (1986) to F. A. Buijtenhuijs et al also reports a similar result that the decomposed products of N-methylmorpholine N-oxide are mainly comprised of N-methylmorpholine and asserts that, when cellulose/N-methylmorpholine N-oxide solution is heated at 120.degree. C. for 16 hours, N-methylmorpholine N-oxide is decomposed in an amount of about 2% by weight in the presence of propyl gallate, an antioxidant, and in an amount of about 5% by weight in the absence of propyl gallate. They also reports that metal ions such as copper and iron ions contained in a spinning dope catalyze the decomposition of amine oxide as well as the cellulose.
According to Zh. Prikl. Khim. (Leningrad), 60(9) 2063-2067 (1987) and Zh. Prikl. Khim. (Leningrad), 61(1) 117-123 (1988), both to A. M. Bochek et al, a cellulose solution in amine oxides such as triethylamine N-oxide, N-methylmorpholine N-oxide and N,N-dimethylethanolamine N-oxide partially decomposes at high temperatures, not only into N-methylmorpholine and morpholine with the N--O bond of amine oxide being broken down, but into olefins.
It is well known that when amine oxides having beta-hydrogen are decomposed at high temperatures, hydroxyl amines and olefins are generated with the oxygen moiety of amine oxide catching the beta-hydrogen. Such reaction is widely utilized for a synthesis path for new olefins (reference, J. D. Roberta and M. C. Caserio, Basic Principles of organic chemistry p 1964).
There has been known no detailed and accurate information for the compositions and components of the coloring impurities resulting from the heat decomposition of N-methylmorpholine N-oxide. Nonetheless, it is believed that even little amount of the coloring impurities tinge the aqueous amine oxide solution reclaimed with dark brown and they are in close relation with the olefin compounds generated on heat decomposition of N-methylmorpholine N-oxide.